11 min read
Every Payment Has a Liability Owner
EMV authorization is the easy part. The hard problems are timeouts, retries, SoftPOS lifecycle gaps, store-and-forward declines, and the financial question that remains when two …
13 pages
Payment-Architecture
11 min read
The €400 Wallet Problem: Concurrency, Lost Updates, and Where to Serialize
Two POS terminals charging the same benefit wallet at the same instant expose the classic lost-update problem. Four serialization strategies — pessimistic locks, optimistic …
10 min read
Manual Capture: The Two-Step Logic Behind Tap to Pay and Modern Card Payments
Manual capture is not about typing a card number by hand. It is the deliberate split of a card transaction into authorization and capture, managed as a backend state machine. Why …
8 min read
Double Charges: The Hardest Problem in Payments Is Remembering You Already Did
A duplicate charge is rarely a coding bug. It is a distributed system losing certainty about whether an authorization already happened — and then guessing wrong. Why blind retries …
5 min read
Where AI Really Fits in POS and E-Commerce Payments
This is part 3 of a series on building grounded AI for payment systems. Part 1 made the case that payments need grounded AI, and part 2 Part 2covered the retrieval pattern that …
10 min read
What Makes a Payment Terminal Secure: SmartPOS vs SoftPOS
In the POS systems I have worked on, “is this terminal secure?” almost never has a hardware-only answer. The card read is one boundary. The PIN entry is another. So are …
5 min read
From Wireless Awareness to Payment Awareness: The RAG Pattern
This is part 2 of a series on building grounded AI for payment systems. Part 1 made the case that payments need grounded AI, not a generic LLM guessing from training data. This …
5 min read
Why Payments Need Grounded AI, Not Just LLMs
This is part 1 of a series on building grounded AI for payment systems. This post sets up the problem. Part 2 covers the retrieval pattern, and part 3 covers the practical use …
8 min read
Partial approvals: what terminals and hosts must do when the card pays only part
A partial approval is not a decline and not a completed sale. ISO 8583 response codes and amount fields must be read together — and the terminal must not show a green APPROVED …
9 min read
When the Issuer Goes Down: STIP, EMV Offline, and Authorization Continuity
When an issuer or its processor drops offline, the POS does not always get a clean decline. The card scheme may stand in and approve or decline on the issuer’s behalf. The …